1. Field of the Invention
The present invention relates to a switching element and a protection circuit using the switching element. More specifically, the present invention relates to a switching element that can switch a current path in two directions and has a reduced chip size, and to a protection circuit using the switching element.
2. Description of the Related Art
As a switching element, a switching element that not only switches a device between ON and OFF but also switches the direction of a current path (the direction in which a current path flows) is also under development, and such a switching element is adopted, for example, to a protection circuit of a secondary battery.
As an example of a conventional two-way switching element, FIG. 5 shows a circuit diagram of a protection circuit for a secondary battery.
A two-way switching element 86 has an overdischarge-prevention switching element 82 connected in series to an overcharge-prevention switching element 83, and a control circuit 84 performs an ON-OFF control.
The control circuit 84 detects the battery voltage and switches the overcharging-prevention switching element 83 off at the time when the detected voltage is higher than the maximum set voltage, thereby preventing a secondary battery 1 from being overcharged. In addition, the control circuit 84 switches the overdischarge-prevention switching element 82 off at the time when the detected voltage is lower than the minimum set voltage, thereby preventing the secondary battery 1 from being overdischarged.
The overdischarge-prevention switching element 82 and the overcharge-prevention switching element 83 have a small internal resistance in their ON states, and are constituted of MOSFETs that can achieve reduced power loss and voltage drop. The MOSFETs have parasitic diodes and, therefore, even when the MOSFETs are in OFF state, a current path can be formed in a desired direction by use of the parasitic diodes.
Therefore, even when the battery voltage becomes higher than the maximum set voltage and thus the MOSFET of the overcharge-prevention switching element 83 is turned off, for instance, the secondary battery 1 can be discharged using the parasitic diodes.
Meanwhile, even when the battery voltage becomes lower than the minimum set voltage and thus the MOSFET of the overdischarge-prevention switching element 82 is turned off, the secondary battery 1 can be charged using the parasitic diodes.
The protection circuit 85 shown in FIG. 5 operates in the manner described above and prevents the secondary battery 1 from being overcharged and overdischarged. This technology is described for instance in Japanese Patent Application Publication No. Hei. 10-12282 (page 7, FIG. 1).
As described above, in the conventional technologies, one of switching elements is set to as the overcharge-prevention switching element 83 for preventing the secondary battery from being overcharged, and the other one of the switching elements is set to as the overdischarge-prevention switching element 82 for preventing the secondary battery 1 from being overdischarged, thereby realizing the two-way switching element 86. Such a two-way switching element 86 is obtained by connecting two switching elements (MOSFETs) of the same size in series, but it prevents miniaturization of size as well as progress in reduction of the manufacturing costs.